The present invention relates to an air/fuel ratio sensor abnormality detecting device for an internal combustion engine. More specifically, the invention relates to an air/fuel ratio sensor abnormality detecting device for detecting abnormality of a downstream side air/fuel ratio sensor in an internal combustion engine which is provided with air/fuel ratio sensors disposed in an exhaust passage at upstream and downstream of a catalytic converter for performing an air/fuel ratio control on the basis of the results of detection by the air/fuel ratio sensors.
Conventionally, in order to improve purification efficiency of an anti-pollution unit, such as catalytic converter disposed in an exhaust system or to improve fuel economy, an internal combustion engine is provided with an upstream side air/fuel ratio sensor at a position upstream of the catalytic converter for performing feedback control of an air/fuel ratio. The upstream side air/fuel ratio arranged upstream of the catalytic converter tends to easily deteriorate due to its being subjected to high temperatures. Therefore, another downstream side air/fuel ratio sensor is provided downstream of the catalytic converter to establish a dual sensor system and to monitor the operation of the upstream side air/fuel ratio sensor to perform correction to better condition the purification performance of the catalytic converter.
Technology for detecting abnormality of the downstream side air/fuel ratio sensor due to deterioration, or the like, on the basis of a difference in the response period between the upstream side air/fuel ratio sensor and the downstream side air/fuel ratio sensor upon fuel cut-off has been proposed in Japanese Unexamined Patent Publication No. 63-239333. Namely, at the occurrence of fuel cut-off in the internal combustion engine, an elapsed time from a time at which the output of the upstream side air/fuel ratio sensor indicates a lean mixture condition, to a time at which the output of the downstream side air/fuel ratio sensor indicates the lean mixture condition so that abnormality is judged when the response timing difference is in excess of a predetermined period. This technology is based on the fact that the response timing difference becomes greater when the response characteristics of the downstream side air/fuel ratio sensor is degraded due to deterioration.
In this case, however, the deterioration condition of the catalytic converter is not taken into account for the setting of the predetermined period giving the reference for comparison. At a certain occasion, even when the response characteristics of the downstream side air/fuel mixture sensor is degraded due to deterioration of the sensor under the deterioration condition of the catalytic converter, the difference of the response timing at the upstream and downstream of the catalytic converter can be held unchanged from that of the normal states of the catalytic converter and the air/fuel ratio sensors. This relates to reduction of O.sub.2 storage effect of the catalyst due to deterioration.
Further discussion will be given for the O.sub.2 storage effect. The O.sub.2 storage effect is an effect that enables storage of O.sub.2 during lean condition of the air/fuel ratio (the condition where the air/fuel ratio is greater than a stoichiometric value) and which discharges the stored O.sub.2 for awhile for promoting oxidation after variation of the air/fuel ratio into a rich condition. This O.sub.2 storage effect is effective for storing O.sub.2 even when the air/fuel ratio is in a slightly lean condition in relation to the stoichiometric air/fuel ratio (A/F=14.7), as shown in FIG. 2. On the other hand, a storage amount of O.sub.2 of the catalyst is at a greater level in new and (not deteriorated) normal catalyst, and is reduced according to progress of deterioration.
Accordingly, when the O.sub.2 storage effect is lowered due to deterioration of the catalyst, a difference of response timing between upstream and downstream of the catalytic converter upon variation of the outputs (from rich state to lean state) of the air/fuel ratio sensors is shorter than that in the normal state of the catalytic converter. This may compensate degradation of the response characteristics of the downstream side air/fuel ratio sensor to make the total response period unchanged from the normal state. As can be appreciated herefrom, deterioration of the catalyst makes it difficult to detect deterioration of the air/fuel ratio sensor and may possibly cause erroneous judgement at a certain setting of the response timing difference. Therefore, there has been a need to provide for accuracy of detection of an abnormality in the air/fuel ratio sensor.